At present, crowns used to repair adult human teeth are custom fabricated according to a mold of a tooth being repaired which is provided by the dental practitioner. The crowns may be formed of stainless steel or other metal clad with porcelain or a ceramic such as zirconia clad with porcelain. Using the mold as a guide, computer software, through a CAD/CAM technology, directs a milling machine to mill a coping or framework of the crown out of the chosen material, such as metal or ceramic, which is eventually clad with porcelain. Although satisfactory in result, the patient receiving such a crown must make multiple visits to the dental office to achieve this result and the process is time-consuming and expensive. The foregoing procedure is particularly difficult when the tooth being replaced is a child's (baby) tooth which will eventually be lost and replaced by permanent teeth through the natural maturing process. Veterinary crowns are also produced in the same manner and can be just as difficult to produce.
Given the difficulties of dealing with a child in a dentist's chair, dentists have developed prefabricated crowns that do not require molds and multiple visits to the dentist. The use of prefabricated, stainless steel crowns for restoring badly broken-down children's teeth is now the standard of care in pediatric dentistry. Recently, the American Academy of Pediatric Dentists has encouraged crown restorations due to the poor outcomes resulting from the use of fillings in certain high-risk groups, especially children with the inability to cooperate in the dental chair, thereby necessitating the use of general anesthesia for their dental treatment. Prefabricated crowns are a very efficient and reliable restoration resource and are the restoration of choice in teeth with moderate to severe dental decay.
Stainless steel prefabricated crowns are the most versatile and widely used crowns in pediatric dentistry. However, due to their unaesthetic appearance, many dentists are turning to other manufacturers of preformed pediatric crowns for a more aesthetic option for their patients. In the late 1980's, the idea of creating a stainless steel crown and covering the visible surface with a tooth-colored composite material veneer started to gain favor. Cheng Crowns (1987), Kinder Krowns (1989), and NuSmile Crowns (1991) are some of the most notable manufacturers to gain significant market share with this revolutionary new crown for pediatric dentistry. Although these new crowns were much more aesthetic than the all-stainless-steel crowns, they came with some significant drawbacks.
The main benefit of the pre-veneered crowns was their more aesthetic appearance compared to their stainless steel predecessors. This new product, however, required a new method for preparing the tooth and seating the crowns. Because the plastic material was bonded to the metal substructure, it was recommended that these restorations have a passive fit to the tooth, minimizing the potential to crack the facing. Crimping or altering the metal substructure, which dentists were accustomed to doing before seating a crown, is not recommended in order to avoid weakening the bond between metal and facing. However, due to the increased strength of modern cements, this passive fit method has become accepted and works quite well with most luting agents used on the market today.
However, because of the interface between the metal and the plastic facing, there is also a tendency for the facing material to crack or chip off the metal substructure. The fragile nature of this interface is a major negative. Dentists often crack a facing when seating the crown or are required to re-treat a returning patient because of an unsightly failed restoration caused by the failure of this weakened interface.
Since the plastic facing must be applied to the front of the tooth over the metal substructure, the facing has a very bulky and bulbous appearance. This is particularly noticeable when the need arises to crown only one anterior tooth. It is very hard to match the contour of the patient's natural teeth when using these bulky crowns, necessitating a significant reduction of the tooth structure in order to ensure a proper fit which can lead to unnecessary involvement of the pulp chamber and the need to perform a pulpotomy on the tooth.
FIG. 11 shows a perspective view of one such bulky crown comprising a stainless steel base 80 and polymeric coating 82.
Matching tooth color is another big challenge when using current aesthetic pediatric crowns. The nature of the plastic overlaying the metal substructure gives them their nickname of “Chiclets.” This is a word that is used frequently by both dental professionals and parents when describing the appearance of the current crowns available on the market.
In the course of sizing the crowns on the teeth to ensure proper fit, it is often necessary to sterilize and restock the unused crowns. The use of heat sterilizing techniques weakens the facing and the overall integrity of the crown. Therefore, it is necessary to use a 24-hour cold sterilizing technique on these crowns because of their plastic facing/metal interface. However, the use of the cold sterilizing technique tends to alter the shade of the facing from its original color. Due to this fact, many offices must maintain a separate storage container for crowns that have been sterilized. As the sterilized crowns will often not match those that have never been tried in the mouth, dental offices must stock a larger inventory of crowns which is a major unnecessary disadvantage for the dentist.
Even with all the potential negatives to these aesthetic crowns, until now they have been the best alternatives for dentists and parents who want a more natural smile for their children.
Various ceramics have been used in dentistry. Alumina, for example, has been used for implant abutments and crown and bridge frameworks and copings. Alumina has more translucency and better matches the translucency of natural teeth, but it lacks in strength and is more prone to failure.
Zirconia formulations have been used in adult dentistry for several years as a replacement for metal for the manufacturing of crown coping or frameworks. The copings are typically layered with porcelain to build up the entire structure of the tooth restoration and to develop the aesthetic surface characteristics. Zirconia has also been used for implant abutments and as endosseous implant cylinders. Zirconia is white in color, and extremely strong. Zirconia has been replacing alumina as the framework material of choice due to its strength. The downside of zirconia is that it is very bright white and by itself, does not match the human dentition well.
Dental crowns and impression systems of various sorts have been proposed in the past. For example, U.S. Pat. No. 6,769,913 describes a device for taking dental impressions. The device includes impression cap having an injection port configured to receive material in an inner cavity.
U.S. Pat. No. 4,492,579 shows a dental crown substrate formed of a noble-based metal formed over a thin metal foil substrate. A veneering material such as porcelain is then coated over the substrate.
U.S. Pat. Nos. 4,992,049, 5,314,335, and 5,538,429 teach dental crowns utilizing a base of metallic mesh or stranded material covered by a veneer of porcelain.
U.S. Pat. Nos. 3,058,216, 3,375,582, 4,392,829, 4,846,718, 5,624,261, 6,106,295 and 7,008,229 describe dental crowns utilizing metal or plastic as a coping which is veneered by porcelain or plastic material.
U.S. Pat. No. 6,663,390 illustrates a ceramic prosthesis in which a metallic core is provided for attachment to an implant in the patient's mouth. A ceramic crown of porcelain zirconia, or polymeric material then covers the extending metallic core.
U.S. Pat. No. 1,609,549 shows a telescopic tooth crown in which the interior of the crown includes a number of indents to aid in the adhesion of the inner and outer shells of the crown.
U.S. Pat. No. 4,766,704 discloses machining a crown from a ceramic blank. United States Published Patent Application No. US2006/0154211 describes a pediatric crown which may be prefabricated in various sizes and shapes for primary dentition. Porcelain has been employed as a outer crown material due to aesthetic appeal.
U.S. Pat. No. 6,638,069 shows a shaping cap using a matrix material including fillers such as zirconia mixed with silica and titanium dioxide.
U.S. Pat. No. 5,775,913 shows an acrylic material filled with quartz or silicon dioxide, called Artglass.
U.S. Pat. No. 6,592,373 shows a crown formed form an injection molded acetal homopolymer.
U.S. Pat. No. 6,974,323 describes a micromechanically acting retention pattern formed by a laser in a zirconia composition implant abutment and dental restoration.
WO 2007/046693 describes a method for increasing the bond strength of stabilized zirconia to another substance.
The disclosures of each of the above references are incorporated by reference herein.
A pre-fabricated strong crown for pedodontic and veterinary applications and process for making the same would be a notable advance in the dental field.